In June, as part of Enbridge’s ongoing inspection process of Line 5 in the Straits of Mackinac, we conducted underwater inspections of the twin pipelines using a remote operated vehicle (ROV) and an autonomous underwater vehicle (AUV). The intent of the inspections was to provide a visual, external inspection of the pipelines to identify potential areas of concern that need additional examination.
Our biennial inspection program worked as designed; we found four sections of pipe that require additional support anchors to reinforce areas where erosion and span distances have increased since the last underwater inspection in 2014. Although the easement agreement with the State of Michigan requires a span distance of 75-feet or less, the average distance between underwater supports along the portion of Line 5 through the Straits is approximately 50 feet.
In the four sections that are slightly longer than 75 feet, the safety or integrity of the Straits Crossing has never been compromised, which has been confirmed by engineering calculations showing that the allowable safe span length is 140-feet. We have contacted the State regarding the results of the inspection and our plans to add additional support. At the same time, we will address another 18 spans before more erosion occurs as those sections have grown over the last two years and are nearing 75 feet.
The Great Lakes is a dynamic environment and we anticipate that at times there can be changes to the lake bottom. In fact when Line 5 was built in 1953, its state-of-the-art engineering design included supports on the lake bed and regular measurements to understand the impact of currents and resulting erosion. In 2014, we installed 40 new support anchors; after that work was completed, there were no spans greater than 75-feet in length. Our robust integrity and inspection programs allow us to identify any areas that require attention, including at the lake bed.
We expect to begin installation of the support anchors in August 2016, pending receipt of the necessary permits from the State.
Enbridge notified the State on July 22, 2016, and applied for permits on Aug. 1. Work was completed on the addition of four new support anchors in November 2016.